I've been looking at the removable floor of Hive A. There is a steady fall of Varroa mites there. I checked three times and the average is about one mite falling per hour. Some were still alive; a few were clearly immature.
This rate of mite drop puts the colony at severe risk of colony collapse, according to the BeeBase document "Managing Varroa".
I have seen quite a few crawling bees around the hive. Some have deformed wings, but not all. I'd be more concerned if this hadn't been the same last year.
We'll see what happens to the colony. My expectation is that it will not collapse, and that this rate of mite drop is seasonal and bearable.
If this colony survives this round of severe Varroa infestation then I'd say they have developed tolerance